The invention relates to new and useful improvements in manufacturing printed wiring assemblies, and more particularly to installation of surface mount and other components on printed wiring boards.
Surface mounting of electronic components on printed wiring boards has become a significant mounting method for printed wiring assemblies used in high quality electronic equipment. This method offers opportunities for reliability and performance improvements of such assemblies and for increasing the efficiency of the manufacturing process. Surface mount technology is explained in an article in OST magazine, October, 1987, at page 15 et seq., which article is incorporated herein by reference.
Surface mount components are often soldered to printed wiring boards using vapor phase reflow or other techniques. In such techniques solder paste is applied to selected pads on the printed wiring board using screen printing or other techniques, and the components are positioned with their terminations atop the solder paste covered pads and are held in place by the paste. The assembly is then passed through a chamber containing the vapor phase of a liquid which has a vaporization temperature higher than the melting temperature of the solder. When the relatively cool printed wiring assembly enters the hot vapor, the vapor condenses onto the printed wiring board and thus heats the printed wiring board to the boiling point of the fluid. The solder is thus melted and joins to the component terminations, and the printed wiring assembly is passed into a cooling chamber where the solder solidifies into its final state.
Application of solder paste using current methods requires considerable effort. The screen printing method requires tedious and time consuming levelling and alignment of the screen printer in order to produce a good deposition pattern free from "skips," bridges and misalignment. Changes in the printed wiring assembly design, or in the amount of solder paste, dictate that a new screen or stencil be fabricated, causing increased expense. Since stencil fabrication is time consuming, long lead times are required.
Surface mounted components, especially small "chip" capacitors and resistors, have a tendency to move laterally and to "tombstone," or stand on end, during the soldering operation. Parts which have moved must be manually relocated, which not only consumes valuable time but also reduces the overall reliability of the assembly. Permanent adhesives have been used to secure components in place to prevent movement, but quality experts are concerned that such permanent adhesives may shorten solder joint fatigue life since such adhesives exhibit a relatively high thermal expansion rate and may add stress or strain to the solder joints during thermal cycling.
Related events also occur when leaded components are soldered to printed wiring boards. Wave soldering is the dominant machine process for soldering of leaded components into plated holes on printed wiring boards. During the wave soldering process, holes in the printed wiring board intended for use and soldering later in the assembly process may be filled with solder unless covered. This requires later reworking to allow insertion of component leads. This problem is typically alleviated by applying a temporary "solder stop" liquid to the board prior to the wave soldering process, which hardens after application and blocks the holes. The solder stop material is water or solvent soluble and is later removed during the board cleaning process. However, the use of such material is time-consuming, whether applied by hand or by automated equipment, and its brittleness, when dry, can lead to cracking and breakage before or during soldering.
In addition, other areas of a printed wiring board may be subject to bridging, shorts and other adverse effects due to exposure of the outside layer of circuitry to the wave of molten solder during the soldering process. To prevent these effects, a polymeric coating, or solder mask, is often applied to the board surface. This mask material is effective, but it is costly and adds weight to boards. In addition, it is subject to its own set of problems, such as misregistration, peeling, inadequate chemical resistance, discoloration and other quality related problems.
It is therefore an object of the present invention to provide a soldering aid which eliminates the need for applying permanent solder masks to the wave-solder side of printed wiring boards.
It is another object of the present invention to provide a soldering aid which temporarily holds down components during soldering to prevent movement and "tombstoning."
It is a further object of the present invention to provide a soldering aid which temporarily masks desired areas of the printed wiring board during soldering.
It is yet another object of the present invention to provide a soldering aid which eliminates the need for screen printing or stencilling solder paste onto printed wiring boards.
It is still another object of the present invention to alleviate solder joint fatigue failure by permitting components to be affixed to printed wiring boards with a controllable standoff distance.